Learn Me: Squish, Domes, Flattops and CCV

Tim Walsh · Posted: Tue Oct 25, 2011 4:35 pm Post subject:

Jim,
In a past life I did run into the insufficient squish results in detonation problem, hence the reason I wasn't too worried about that, especially considering how light the pock marks are on the old piston. I'll bet I could count them without taking my socks off.

From the research I've done there is definitely a difference between the three popular pistons, the 2005 CR, the 1999 CR and the RS.

I took another measurement today and got 0.055"

Further reading:
http://www.ekartingnews.com/viewtopic.php?t=69479&highlight=
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2002 Birel CR32V/ StockMoto w/PI

Larry Andrews · Posted: Tue Oct 25, 2011 10:51 pm Post subject:

Pock marks *can* come from sand, etc... getting sucked through an unfiltered airbox. (ask me how I know about this) Way more likely due to detonation...but it's important to keep all possibilities on the table when you're in the dark. There's a Feynman quote that goes something like 'The most important thing is not to fool yourself and yourself is the easiest person to fool'.

Let's try this measurement next. Put the barrel on the motor with the base gasket and try to figure out how far the piston stays below the rim of the barrel. It should be pretty close to the rim. I might be willing to burn a head gasket on my stocker to do the same thing for comparison data. Alan's idea of an oddball rod in the motor is a distinct possibility but the only alternative I know of is a Wiseco HotRod and they should be identical dimensionally. Not many people have the tooling to rebuild a crank anymore, but it's not hard to do and entirely possible.

The thing I can't figure out is where does the additional clearance come from. If the barrel is cut anywhere, it takes away clearance. Cutting the head *usually* reduces clearance but it is possible to add some by cutting into the head. (Shim under the plug?) Short rod could do it, but where would that rod come from? <scratching head> Interesting problem...

Tim Walsh · Posted: Wed Oct 26, 2011 7:36 am Post subject:

After discussing it with the kart owner(otherwise refereed to as "my wife"), we're going to leave it as is until the end of the season. After our Dec 11th event, I'll pull the head and inspect it to see if it was cut for an RS piston, and fix the squish. Is there a certain CCV I'm looking for for a modified motor? I realize that stock moto shoot for just over 11cc. I think if I want 14:1 compression I'd be shooting for 8.9cc?
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2002 Birel CR32V/ StockMoto w/PI

Christian Hubbell · Posted: Wed Oct 26, 2011 1:43 pm Post subject:

I bought one of these to take measurements: http://www.shopswedetech.com/product_p/cr125%20modified%20top%20end.htm

If you want the measurements or even the piston itself let me know, I'll make you a deal.

Tom Reynolds · Posted: Wed Oct 26, 2011 1:47 pm Post subject:

That's just an 2005 CR125 piston topend kit isn't it?

Tim Walsh · Posted: Thu Oct 27, 2011 4:59 am Post subject:

Looks like it is just a 2005 piston to me based on the 0.040" piston height. What I could really use is someone to make an accurate measurement of the pin height on the '99 CR piston and compare it to an RS or '05 CR piston. This would confirm my RS to CR piston swap.

Christian,
Does it look like this?

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2002 Birel CR32V/ StockMoto w/PI

Tom Reynolds · Posted: Thu Oct 27, 2011 7:19 am Post subject:

Yeah I've used the 05 instead of the RS before. Only thing I can tell you is my very easy going engine builder who often gives alternative parts really prefers the RS. It's one of the few things he doesn't really like alternatives and claims its .5hp over the 05 on his build. Not saying don't do it, but just a point of reference.

Larry Andrews · Posted: Thu Oct 27, 2011 8:06 am Post subject:

20.46 - Brand new '99 piston.

al nunley · Joined: 13 Nov 2006 Posts: 3066

I didn't see where anyone asked what the CC's are, or will be, with any of these changes. Seems to me a very important thing to know.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

Tim Walsh · Posted: Thu Oct 27, 2011 8:24 am Post subject:

So if I'm doing the math right, I should end up with ~ 0.006" of squish if I just slap an RS piston in it, and would require about 0.026 of additional shim. This additional shim would or would not alter the port timing? As I think of it the new piston would effectively move the piston up in the bore, and the shim would move the cylinder an equal amount(equal to a correct flattop installation), and thus similar port timing?

Then in theory, all that the RS piston would do is correct my squish issue, and then reduce my CCV. How little is too little when it comes to CR/CCV?

Edit: If I put in a 2005 CR piston, I think I'd end up near 0.035 for a reasonable squish, but more CCV, and the ports lower in the bore? I'm not sure what that would result in, but in my brain that would be the same as changing the camshaft in a 4/ engine.
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2002 Birel CR32V/ StockMoto w/PI

Christian Hubbell · Posted: Thu Oct 27, 2011 9:44 am Post subject:

Ya, pretty sure it's a 2005 dome top. I'll grab the Honda part number tonight to be sure. I did take all the measurements but the hard drive it was on stopped working the other day Sad

I am going to try some recovery measures today.
I'm not sure why you said you would have more CCV with the 2005 over your flattop but I'll go back and read.

As another wrench in the mix I like the Wiseco racers choice piston Wink

Christian Hubbell · Posted: Thu Oct 27, 2011 10:00 am Post subject:

If some one has an old 99 CR piston I'll measure it up, think Larry's got it though. The rs has a taller dome over the 05 so compression would be higher if the squishes were the same.

Tim Walsh · Posted: Thu Oct 27, 2011 10:08 am Post subject:

Tim Walsh · Posted: Thu Oct 27, 2011 12:24 pm Post subject:

Jason Vehige · Posted: Thu Oct 27, 2011 1:49 pm Post subject:

There are times I envy you Moto guys... but I must say reading this kind of stuff makes me happy I just call Russell and have him send me an SGM SL204 top end kit... Laughing

All kidding aside... good luck Tim ... there is more than enough advice available here to get your squish and CCV settings where they need to be without breaking the bank, making a good bit more power will be a nice bonus too!!
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Jason Vehige

08 Gillard Charlotte / SGM sl204
05 Vanspeed Sirio / SGM sl204